1. Field of the Invention
The present invention relates to anode foil used in an aluminum electrolytic capacitor, and particularly relates to aluminum foil used as anode foil of an aluminum electrolytic capacitor for middle to high voltage of 200 V or more and a method for manufacturing the same.
2. Background Art
In recent years, as electronic apparatuses have had a smaller size and higher reliability, makers of electronic apparatuses have strongly required reduction in size of an aluminum electrolytic capacitor. Therefore, electrode foil used in the aluminum electrolytic capacitor is also required to have a larger capacitance per unit area than that of a conventional one.
A general aluminum electrolytic capacitor has a capacitor element formed by winding anode foil and cathode foil with a separator sandwiched therebetween. The anode foil is produced by etching aluminum foil so as to enlarge an effective surface area, and forming a dielectric oxide film on the surface of the aluminum foil by anodic oxidation. The cathode foil is produced by etching aluminum foil so as to enlarge an effective surface area. The aluminum electrolytic capacitor is formed by impregnating the capacitor element with an electrolytic solution, and sealing the capacitor element in a metal case.
In order to enhance a capacitance or to reduce the size in an aluminum electrolytic capacitor, it is essential to enlarge an effective surface area of anode foil and to enhance a capacitance per unit area. Therefore, techniques for etching and forming treatment to enlarge the effective surface area of anode foil are being developed actively.
In general, aluminum foil used to produce anode foil is chemically or electrochemically etched in a hydrochloric acid aqueous solution containing acids for forming a film, for example, sulfuric acid, nitric acid, phosphoric acid, and oxalic acid. In particular, an etching treatment method for aluminum foil used at middle to high voltage includes a preceding-stage etching step and a subsequent-stage etching step. In the preceding-stage etching step, basically, tunnel-shaped pits (hereinafter, simply referred to as “pits”) are formed. In the subsequent-stage etching step, each of the pits is enlarged so as to have a diameter suitable for a voltage at which an aluminum electrolytic capacitor is used. In this treatment method, it is important how the many pits are generated and how efficiently the pits are enlarged.
In such an etching treatment method for aluminum foil used at middle to high voltage, techniques for efficiently forming pits are roughly classified into a surface modification technique and an etching treatment technique of aluminum foil.
As the surface modification technique of aluminum foil, it is generally known that the ratio of crystals occupied in the Miller index (100) plane of aluminum foil is made to be 80% or more (see, for example, Japanese Patent Unexamined Publication No. H10-81945).
Furthermore, Japanese Patent Unexamined Publication No. H6-124855 proposes a technique of allowing one or more kinds from Pb, In, and Sn to be present in an amount of 100 to 5000 ppm in total in a surface layer part from the surface to a depth of 0.1 μm of aluminum foil of an electrolytic capacitor, and to be contained in an amount of 1 to 5 ppm in total in the inside part excluding the surface layer part. This technique aims to form a large number etching pits efficiently by allowing the etching to proceed to the inside of the aluminum foil with the surface dissolution of the aluminum foil suppressed. As a result, it is mentioned that the area enlargement ratio is increased and the capacitance can be increased.
In addition, Japanese Patent Unexamined Publication No. 2003-229334 proposes a technique for uniformly providing carbon on aluminum foil by, for example, vapor deposition, and then giving energy to react aluminum with carbon so as to form Al4C3 particles. This technique aims to disperse and distribute appropriate number of etching pits uniformly from the introduced Al4C3 as a starting point of an initial pit and by preventing the starting point from dropping off. As a result, it is mentioned that an electrolytic capacitor with a large capacitance can be obtained.
However, in these surface modification techniques, even if Pb and Al4C3 are attached, the capacitance is not so enhanced. This is because concavity and convexity of damages formed by being roll-pressed (hereinafter referred to as “roll-pressed damage”) remain, which are generated on the aluminum foil surface when an aluminum material is roll-pressed. Pits are surely generated from a portion to which Pb and Al4C3 are attached, but pits are also generated from the concavity and convexity of the roll-pressed damage. Therefore, pits are further generated to link a plurality of pits, which have been generated uniformly, to each other. As a result, the number of pits is reduced. That is to say, joining of pits occurs.
Furthermore, Pb and the like are concentrated in the vicinity of the surface, and particularly in a roll-pressed mark. Pits are easily generated from the positions of the concentrated Pb. Therefore, pits are generated in a narrow range so that they are overlapped with each other. As a result, joining of pits occurs.
Furthermore, Japanese Patent Unexamined Publication No. 2008-231512 proposes a technique of forming a metal layer having a plurality of crater-shaped recesses and made of at least one selected from the group consisting of Cu, Ni, Co, Fe, Mn, Mg, Zn, Pb, Bi, In, Sn, and Sb on the surface of the recesses. It is mentioned that by making the average size of the opening of the plurality of crater-shaped recesses in the range of 0.05 μm to 5 μm, a high capacitance can be exhibited when etching is carried out.
However, in order to form the craters, it is necessary to form a porous oxidized film by carrying out anodic oxidation treatment and to remove the porous oxidized film. In this technique, since a metal layer is formed on aluminum foil by, for example, vapor deposition, the steps are complicated. Therefore, this technique is not suitable for practical use.